The present invention relates to a method and an apparatus for the analysis of chemical constituents in an electrolysis cell for production of metal. In particular, the invention relates to continuous analysis of the oxide concentration and the cryolite ratio in electrolysis cells for production of aluminum working in accordance with the Hall-Hxc3xa9roult process.
In the Hall-Hxc3xa9roult process, aluminum is produced by electrolytic reduction of alumina (Al2O3) dissolved in a bath based on molten cryolite (Na3AlF6). The metal is formed at the molten aluminum cathode, and oxygen is discharged at the carbon anode. Increasing knowledge of the processes involved, has enabled modifications to be made on the bath compositions. Some fluorides in small and limited amounts are added to the bath in order to lower the bath temperature, and increase the efficiency of the electrolytic process. Aluminum fluoride (AlF3) is the most common additive, and commonly cells are operated with an excess aluminum fluoride content relative to cryolite. In addition, the bath may contain a certain amount of calcium fluoride (CaF2), which mainly originates from the calcium oxide (CaO) impurity in the alumina feed. In some cases, calcium fluoride is voluntary added to the bath as well.
To operate the cells in a manner that is effective with respect to several criteria such as dissolution of alumina, energy consumption, current. efficiency, sludge formation, compensation of additives as a result of evaporation losses and losses when draining metal, environmental regulations, etc., it is of great importance that parameters such as the cryolite ratio CR (the molar ratio of NaF and AlF3), bath ratio (the mass ratio of NaF and AlF3), excess AlF3 (the mass % AlF3 in excess of the Na3AlF6 composition) and the A oxide concentration can be determined in a precise and continuous manner.
Up to present, numerous suggestions have been made as to possible structural species in cryolite-alumina melts as such. In this work, Raman spectroscopic measurements have been performed to analyse the bath composition. Such measurements are based upon the fact that the different species emit light at a characteristic wavelength, and the technique is commonly used to indicate the presence of different species in laboratory work. One example of such work performed is published in the paper: xe2x80x9cReinvestigation of Molten Fluoraluminate Raman Spectra: The Question of the Existence of AlF52xe2x88x92 Ions, B. Gilbert and T. Materne, Applied Spectroscopy, Volume 44, Number 2, 1990xe2x80x9d. In addition, the paper discloses a laboratory equipment for spectroscopic analysis of melt samples.
Further, one commonly used method for indicating the alumina concentration in an electrolysis cell is based upon. cell voltage measurements related to the electrical conductivity of the electrolyte. Other methods involve taking samples of the bath for the analysis of the composition in a laboratory.
However, present methods are either not very precise or not continuous and need further evaluation to fit the steadily increasing demands, of the electrolysers for optimising their processes.
In accordance with the present invention, it is now possible to determine both the alumina (oxide) concentration and the cryolite ratio in a precise and continuous manner. in an electrolysis cell under its production of metal. The invention involves the use of Raman spectroscopy where spectrums of light emitted from the melt/bath are compared to reference spectrums generated from samples of known compositions. This xe2x80x9cfingerprintxe2x80x9d recognition. method has proved to be very accurate, and the apparatus together with the proposed method make it possible to perform the analysis in an continuous manner.